Magnetic materials and methods of making the same



United States Patent MAGNETIC MATERIALS AND METHODS OF MAKING THE SAME Reginald S. Dean, Washington, D. C., and John D. Burney and William 0. Cook, Indianapolis, Ind., assignors to P. R. Mallory & Co., Inc., Indianapolis, Ind., a corporation of Delaware No Drawing. Application September 19, 1951, Serial No. 247,376

1 Claim. (Cl. 252-625) This invention relates to magnetic materials and has particular reference to such materials including means and methods for providing soft magnetic articles.

The prime object of the invention is to provide magnetic bodies having high initial permeability, low coercive force, and high electrical resistance.

Another object of the invention is the providing of cores for coils to be used at high frequency.

Other objects of the present invention and advantages thereof will become apparent from the following description.

Previous to the present invention, there have been efforts to use oxide magnetic materials for this purpose. The essential ingredient of such oxide materials is iron oxide and one of the earliest materials tried was the magnetic oxide of iron, ferric ferroso oxide (FeaOr) which occurs naturally as magnetite. This oxide is highly magnetic and, by suitable heat treatment, it can be obtained with low coercive force. Magnetite, however, is a good electrical conductor and is not suitable for use at high frequencies on account of the eddy currents set up in the core.

Other attempts were made to modify the composition of magnetite so as to retain its magnetic characterics and, at the same time, produce high electrical resistance. This modification most frequently took the form of replacing the ferrous iron in magnetite with another metal. This resulted in largely eliminating the electrical conductivity which is thought to be due to a transference of electrons along a chain of alternately ferrous and ferric ions in magnetite. Among the many ferrites which have been used are cobalt, nickel, zinc, manganese, copper and magnesium. Some of these ferrites have magnetic and electrical properties which make them useful for high frequency cores within certain limited ranges.

Recent developments in the art disclose mixed crystal ferrites, that is, a ferrite material comprised of two or more ferrites, a ferrite being a compound of metal oxide and iron oxide having the composition MFezOr where M is a bivalent metal. It is, accordingly, clear that in such materials iron is present entirely in the ferric state. When this is the case, the electrical resistance of cores made from the materials is very high. However, in order to obtain efi'ectively high permcabilities for many uses, it is necessary to heat to high temperatures whereby some oxygen is lost. This loss of oxygen results in the formation of some ferrous iron and a consequential decrease in electrical resistance which makes such cores unsuitable for higher frequencies.

Another approach to the problem of suitably modifying the composition and properties is to introduce vanadium into the magnetite molecule. This has been done by oxi dation of the compound FeVO. On oxidation of this compound, a magnetic material is formed having in its final state of oxidation the empirical formula (FeV)3O4 and the crystal structure of magnetite. This material is magnetic and has low coercive force and high electrical resistance. The magnetic oxidation product of FeVO can be combined with metal oxides, such as MgO, without alter ing its structure or its basic electrical or magnetic characteristics. These materials, however, do not possess a sufficiently high permeability to make them generally useful as magnetic cores.

The problem which is solved in the present invention is the improvement of the permeability of iron-vanadium oxides without loss of the high resistance characteristics which make them valuable for use at high frequencies.

It has been found that the permeability of these materials increases with the ratio of Fe:V in the compound and that this ratio may be as high as 11:1 without introducing objectionable electrical conduction.

One method of preparing the materials of this invention is to react the compound FeVO with FezOs. This reaction may be considered to proceed in accordance with the following equation:

(shaf It is not necessary, however, that the reaction be carried out in this proportion since the compound FeVO forms a magnetic material isomorphous with magnetite even on slight oxidation and oxygen may be added to the lattice without change in parameters and, therefore, presumably interstitially. FeVO and FezOs may therefore be reacted in any desired proportion to yield a magnetite lattice in which vanadium is substituted for part of the iron. Although the exact valence relationships in these compounds has not been determined, it will be seen from the stoichiometric considerations considerations that some iron must be in the ferrous condition and the compounds cannot be considered vanadium ferrites of the formula VO-FezOg, which in point of fact are not known to exist.

It has been found that the best magnetic properties are obtained by mixing FeVO and F6203 in the proportions of approximately 3:8, although useful materials are obtained over a wide range of proportions. In fact, the introduction of vanadium into the magnetite lattice in any proportion increases its resistance.

In Table I are given the magnetic properties of cores made from various proportions of F8203 and FeVO.

TABLE I Magnetic properties of FezOs-FeVO cores Frequency 82 Percent Ratio Fevo Fezoa 400 kc. 3 me. 25 me.

(Q) I fl. QH efi. IQ! I. efi- It has been found that the addition of certain oxides improves the properties of the ferrovanadium oxide magnetic materials. These oxides enter into the lattice in an isomorphous fashion without changing the crystal structure. The oxides which we have found useful are zinc oxide, magnesium oxide, nickel oxide and manganese oxide. The effect of these oxides is, among other things, to change the Curie temperature. Materials having different Curie temperatures are useful for different purposes. For highest initial permeability at room temperature, a low Curie temperature is desirable and for this purpose we prefer to add zinc oxide.

Experiments carried out to show the effect on the Curie temperature and effective permeability of adding zinc oxide to ferrovanadium oxide reveal that the loss factor is substantially unchanged by the addition of ZnO.

The effect of nickel oxide on the ferrovanadium oxide materials, on the other hand, is to raise the Curie temperature and thereby give the apparent effect of lowering the initial permeability. However, if the materials containing nickel oxide are quenched from just below their Curie temperature, good properties are obtained.

The efiect of manganese and magnesium oxides is less pronounced than are those of zinc or nickel. Magnesium oxide is sometimes added incidentally to the method of preparation and its effect is in no sense deleterious.

The losses in oxide magnetic materials are influenced by heat treatment and in ferrites it is frequently necessary, in order to obtain the desired permeabilities, to heat to temperature where oxygen loss increases eddy current losses. This difiiculty is only partly overcome by the expedient of heating in oxygen. With ferrovanadium oxides, the maximum permeability is reached at temperatures below those at which oxygen losses are encountered. In general, this temperature is about 1100 C., althoug it may be influenced by composition and processing.

In Table II is shown an example of the effect of sintering temperature on permeability and losses of a ferrovanadium oxide magnetic material having the composition of FeVO 2MgO 6Fe2Os -ZnO.

One method of making the ferrovanadium oxide magnetic materials of this invention is to heat the compound FeVO with FezOs and other oxides. The FeVO may be made by the reaction of FeCla and VOCla in equimolecular proportions in ether with CHsMgCl to give FeVO (CII3)6 which is decomposed by heating to form FeVO. However, other procedures may be more economical and particularly the reaction of iron and vanadium oxides with magnesium in proper proportion. For example,

V203 +Fe2O3 4Mg- 2 (FeVO 2MgO) The product of this reaction containing MgO can be used for the practice of the present invention.

Other reactions may be employed for obtaining the proper metal to oxygen ratio for FeVO such as mixtures of the metals and their oxides in suitable proportions. The materials for this invention can also be made directly by mixing all ingredients in the proper proportions and heating until reaction takes place; thus FezOs, V203, zinc oxide and magnesium may be heated until reaction is complete. Similarly, FezOs, V and ZnO may be heated. The following examples are given as illustrative of the invention:

Slug core results a Wet mixing was used. The oxide powders were weighed and water added before mixing with a mechanical stirrer to give the approximate consistency of SAE 30 oil. Two drops of Aerosol as a wetting agent were added and mixing continued for at least 30 minutes. The mix was then dried in an oven at 110 C. overnight. It was then pulverized to pass a 60 mesh sieve. To this powder 8% by weight of a 5% soluble starch solution was added. After mixing, the powder was screened through a 30 mesh sieve. The mix was then pressed into the desired shape ready for sintering. This particular core was sintered in air for 3 hours at 1250 C. The rates of heating and cooling were slow.

It is to be noted that the examples given here are purely illustrative and that the magnetic materials of this invention may be formed by any of the known ceramic methods, such as dry grinding and pressing, wet grinding and pressing, slip casting, vacuum pugmilling and extruding, and the like. The combination of the oxides may be brought about by presintering before or after pressing v or by pressing, sintering, regrinding and repressing.

These and other changes may be made within the spirit of the present invention without departing from the scope thereof, which is not to be limited except by the character of the claim appended hereto.

What is claimed is:

The method of making a soft magnetic material comprising the steps of weighing out the following percentages by weight of 61.8% FezOs; 15% ZnO'; 23.2% FeVO, adding water thereto, mixing the same to an approximate consistency of SAE 30 oil, adding two drops of a wetting agent, continuing said mixing for at least thirty minutes, drying said mix in an oven at 110 C. for a period of approximately fourteen hours, pulverizing the same to a powder adapted to pass through a 60 mesh sieve, adding 8% by weight of a 5% soluble starch solution, pressing the same into a compact, sintering the same in air for three hours at approximately 1250 C., and allowing the same to slowly cool thereafter.

References Cited in the file of this patent Arch Eisenhuttenw, 12 pages, 323-8. (Copy in Patent Ofiice Library.) Abstracted in C. A. v. 33 2091. 

